How to make rounded faces to model a human face?
Edges are by definition straight, and faces by definition planar. A strip of edges or faces can be curved by offsetting the direction of each a tiny bit to the former.

Luckily, you don't (in most cases) have to do this by hand. For examples of how it works and looks, try out the sculpting tools sometime!

EDIT as Duane Dibbley points out below, it IS in fact possible to curve faces that have more than 3 vertices but it may cause some rendering oddities. The reason I did not consider it is because most applications I import models into accept tris (tri-verticed faces) only. Check your target application (if any) before attempting this.